I would think medical gauze would work if the weave is tight enough-holes aren't too large.

olychick, the seeds are soaked (in the cheesecloth) along with the lemon sliced into small pieces. So it would be very hard to fish out all the seeds.

Actually, a coffee filter might work, if you can close it. And hose was a great suggestion...not that I think I've worn hose in the 21st c. Do you have a plastic strainer or mini-colander? An herb sock, like for bouquet garni? Of one of those things that goes in the turkey? A lemon herb teabag? Ease open, dump tea, tie in seeds with the string. even a baggie with holes in it,...

As soon as you add the sugar, any loose seeds will float on the top so you can easily scoop them out.

Linnea, I was imagining pulling the lemon slices out after everything was soaked, then straining the liquid to remove and squeeze the seeds.

Oh, I have a lingerie bag that could work, too, if you might have one of those (used to launder delicate items). Or a jelly bag.

How about a herb/vegetable bag that you might reuse from the grocery store? The weave would be fine enough and be food safe.

Kind of late now, but adding my 2 cents anyway.

Untreated cotton gauze should work. I have lots of muslin and would probably use that. Any untreated cotton fabric should work, but what about just soaking the seeds separately, in some of the water set aside from the lemon slices, nd then straining it back into the fruit mixture?

I use paper towels (instead of coffee filters) to line a large stainless steel strainer and then pour whatever I am straining into that. The liquid filters through the paper towels much more quickly than it does through coffee filters. I then fold the paper towel over whatever has been strained and press on the contents with a rubber spatula (or my fingers) without removing the paper towel, which needs the support of the strainer.

This is what I do with spices and herbs that I use for making my tonic syrup.

It doesn’t have to be cheese cloth. Any bit of fairly loose weave fabric will work. I use old pillowcases for jelly and marmalade making but I’ve also used handkerchiefs and muslin squares. Coffee filters are too impermeable and paper falls apart if soaked. It’s not just for straining. It needs to withstand boiling, soaking and squeezing.

I opted for the coffee filter, 2 layers, tied with string. The seed bag is removed before boiling, so it probably won't fall apart too quickly. I only got about a tablespoon's worth of seeds out of 5 Meyer lemons, maybe a little less, so I hope that will be enough to provide the natural pectin needed.

If this recipe works well, and I want to repeat it in the future, I'll get some muslin next time I'm in a fabric store.

I buy cheesecloth at the grocery store. It's better than muslin for straining purposes, IMO, and last time I got some, cost less than muslin.

I realize now that I read the question wrong, and so I will tell you now what I do in your situation:

I have made a bunch of little bags out of chiffon and organza scraps that I have in my sewing room using my overlock (serger) machine, mainly to use for holding bouquet garni, but they would be good for holding lemon seeds also. When I am using them, I tie the top closed with a piece of string, and this string also allows me to remove the bags when I want to.

If you do not have chiffon or organza, you can use cotton batiste, lawn, or voile. The suggestion to use pantyhose is also good, but I don't have any of those.

Anyway, I wanted to let you know that you can use chiffon, organza, batiste, lawn, or voile if you do not have cheesecloth, and these fabrics will work very well. I do like to serge the raw edges of these fabrics, however, since they are a somewhat loose or open weave.

Another fabric that would be good is lino weave, which is often used for sheer curtains, although I do have a couple of shirts from the 1950s made of this fabric.