I downloaded the 8x10 manual from the Harbor Freight site, and compared it to my manual for the 10x12. All parts appear identical, except for differences in sizes and parts numbers, so my expectation would be you'd have similar design weaknesses that we've experienced in the 10x12. The manuals say the 10x12 is 123" tall, and the 8x10 is about 87" tall, so I guess the 8x10 is a bit less of a target for winds...but if I was building one, I'd probably do similar modifications like we did for my 10x12 (cautious me!)

I can't find any explanation in the 8x10 manual for the white front panels shown on the Harbor Freight link you provided. You might call the HF technical assistance number, 1-800-444-3353 to ask. The 8x10 manual specifies "transparent polycarbonate sheets" just like the manual for my 10x12, same thickness (0.165) so I'm assuming they use the same 4mm twinwall polycarbonate, but knowing Harbor Freight, they may be supplying something else for the front panels...and then just not mentioning it in their website or manual. I'd ask.

Did you find this two-part thread about some Harbor Freight owners experiencing early panel deterioriation problems? Since your climate is similar to mine (some days over 100°) I wanted to be sure. I still think a modified HFGH is a good deal, but some of us in hot climates may find our HFGH panels don't last as long as folks in cooler climates. The person who started this thread (Laserfan) is located in Texas.
HFGH Polycarb Panels failing!

Some folks supplement their panels with bubblewrap for extra insulation during the winter. I didn't caulk my panels because I like to be able to remove them, for replacing with screens, etc. I did use foam weatherstripping around the edge of each panel. That really slowed down the panel installation process, but I find it gives me a very good air seal around the panel edges.

I think dropping the interior temps to 30° below outside temps will be a challenge, and I agree your only hope will be some kind of air conditioner or evaporative cooler, as you mentioned, and perhaps a misting system too. (The smaller the greenhouse, the harder it is to keep cool, and I think the extra height of the 10x12 probably helps a bit with my cooling challenge.) Hopefully someone else has some ideas on that point (I grow cacti so I'm not much help in that department.) ;-)
Sheri

Thanks for the informative post. I have read about the panels deteriorating in the heat. I'm turned off by that and deciding if I want to take my chances. The location I have picked out for it will probably get full sun late morning to mid-late afternoon. Other times it will be shaded by trees. I like the foam insulation idea for replacing panels in the future.

I'm also looking into some of the more expensive units. Is there any benefit to having the crystal clear panels? I imagine they let more light in. Will it make the inside hotter and harder to cool, or might they insulate better? Do they hold up well over time, or do they yellow?

By "crystal clear panels" I'm thinking you mean single layer panels (not twinwall poly.) I believe the main downside to those is they will not insulate as well against the cold as twinwall polycarbonate; anything made from one layer (glass or some type of plastic) doesn't have the insulating value of a twinwall or triplewall panel, since the latter has small pockets of air sandwiched in between the layers.

Here's a heating calculator that might help. It assigns different heat loss values to different types of greenhouse panels. According to their chart, a single layer of fiberglass or polycarbonate has a heat loss value of 1.20. 1/8" glass has a heat loss value of 1.13. But, even the thinnest twinwall polycarbonate (4mm) has a much lower heat loss value of .70. This doesn't mean you shouldn't consider single thickness panels...just that it will cost you more to heat, since they can't retain the heat as well...so just another factor to consider.
Heat Loss Calculator

Twinwall polycarbonate allows plenty of light. In fact, most folks find they need to use some sort of shadecloth to avoid sunburning plants, even with twinwall poly, at least part of the year, but this can vary with climate and things like the shade trees you mentioned. I would think glass or "crystal clear" panels would have a slightly higher chance of plant sunburn, since they would not diffuse the light as well as twinwall polycarbonate.

BTW, don't let the questionable performance of the HFGH twinwall polycarbonate panels scare you away from twinwall poly as a good greenhouse material. I really think the Harbor Freight panels are exceptionally poor (or lacking entirely) in UV protection, but most quality greenhouses with twinwall polycarbonate panels don't have the problems you read about in that "failing HFGH panel" thread.

I know this can all be a lot to absorb, but if you keep asking questions, and reading archived posts here, you'll find a lot of good information! :-)

I am very happy with my HFGH, no sign of failing panels after three years and it just survived a record snowfall and very high winds.

Anyone out there using a HFGH down south in the temperature range I am in? I'm starting to wonder if my cooling goals are even achievable (20 to 30F below ambient) and how this might affect my electric bill. Any experiences would be appreciated!

Also, would a 6x8 be easier/cheaper to cool/heat than the larger sizes? I don't mean by cost per square foot, where the larger sizes might win, but just overall cost.