I absolutely swear by the XS peep sights I put on my Renegade. They aren't cheap at around $100 but they are some of the best you'll find (buy cheap and you get cheap - over and over again). Couple of buddies put Lyman peep sites on their guns, one a Knight the other a Cabelas side lock and they are satisfied.

Can you get them in different aperture sizes? What size do you suggest?

What type of ML are you using?  I have a T/C Triamph that I put a Wiliams peep on and have had no problem with it and plan to use it this year over in Utah for the ML hunt that starts in a couple of weeks.  I did have to modify it a little to get it to fit around the barrel but everything that I had to grind down is on the inside of the sight and you can't see it, and with the sight being aluminum it won't rust.  Now on my Renagade .50 and .54 calibers barrels I am leaving the open sights on them. 

It's is the pits to get old and have problems with your eyes but next month that is getting rectified. 

I shoot a Thompson Center Omega Z5. Does that peep come in different aperture sizes? What sizes do you suggest?

For hunting purposes use the largest diameter peep you can get.  They're quicker to use, work better in low light conditions and less likely to get plugged at the wrong time - doesn't happen often but always happens at the wrong time (I've seen them get plugged with rain drops & snow).

Several makes have adjustable or interchangable peep sizes; small for target use, large for hunting.  A friend using a Lyman peep unscrewed the peep apeture out of the sight while cleaning the gun, then misplaced it.  He said the hell with it & shot the gun without it.  We coulnd't see much if any difference in group size at 100 yards & he's been hunting / shooting without for a couple of years now.  He found it later but never saw the reason to put it back on.

Hawkeye, what sight did you decide to get? I am looking to replace my fiber optic on ProHunter for Co hunt this fall.

Another option.

http://www.midwayusa.com/product/715669/williams-wgrs-cva-guide-receiver-peep-sight-cva-rifles-with-round-barrel-and-winchester-x-150-muzzleloaders-aluminum-black