Here's a design for a mulible use rack that holds 12 trays made from 1"PVC pipe. With the use of PVC pipe you can also use it outside, covered in plastic makes a mini-greenhouse.
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Seems some of my message was lost. Now this rack is 6'6" tall by 2'x4'wide. The whole thing can be picked up with one hand. It holds 954 seedling, 150 32 oz cups. I have 5 of these going right now. I use 6 40watt kitchen and 6 40watt cool white tube to correct color and light balance. much better than daylight tubes and stronger light. I have four of these in an unheated room covered in plastic. The lights keep the temp around 80 degrees all the time. I use a cup and vinger to make co2 so I don't have to visit them for 48 or 72 hours.

i had built a PVC rack several years ago

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but it was not as sturdy as the wooden one and you can't use PVC outisde for more than a year or two as it becomes brittle from exposure to sunlight.

you said you use a cup of vinegar to make CO2. how do you keep the CO2 contained? un-aided it will "flow" like water to the lowest point in its "enclosure" and sit there. so the plants on the floor or possibly bottom shelf will be getting some [minimal] effect from it but the plants above knee height will never get more than a passing glance of CO2.

Interesting, never had a problem with the PVC pipe getting brittle over 6 years now. As for the co2. I have my structures inclosed with 6 mil construction plastic. That only last about a year outside. 18 months inside. I have a small 7" clamp fan for air circulation and a water bowl for humitity. You can buy used computer fans but I like the air circulation better with the larger fan. The weight is handled by using a middle pole. The shelves are made from plywood cut into 2'x4' pieces. I don't glue it, just drill hole at all joints and use plastic wire ties to join it all together. I simply visit construction sites for the materials. Shake a few hands and get plenty of left over scrap. The concrete forms they lay floors with are plywood. A piece gets busted they throw it away and make another. I get the PVC, wire for the electric, pipe glue, etc all from construction sites. The tubes I get from janitors at government buildings, schools and businesses. The fans are from the yearly Spring cleaning trash day we have. In fact we have another one coming up next month. Best thing is they can be taken apart and stored.

kudos to those of you who are good with saws and carpentry and stuff. I don't have the inclination or the patience, so I "built" my seed starting racks by getting one of those 5-shelf (about 6' high) wire rack setups from Target, two plug-in shoplight fixtures from Lowe's, a little bit of smallish chain and some s-hooks also from Lowe's and the appropriate bulbs. After I banged the shelves together according to the instructions, I simply hung the lights using the chain and s-hooks. It could be a better setup, but it was definitely easy and I can take it apart if I tire of seed starting (yeah, riiiiight...)

Lisa