Cannot say enough how bad of an idea using Velcro is on anything that gets above 120F - it turns to goo and just falls off. Makes a lot of static, and leaves a really sticky mess behind.
If I did own a 2011iMac, and I did change drives - for an SSD, I'd wipe down some aluminum surface with a bit of acetone, and if the replacement drive was not painted - that too, otherwise some isopropanol to wipe down the replacement drive. I'd make sure the drive works 100%, and if it did, I'd then use some 3M VHB Grey tape to bond that drive to the wall of the iMac.
Yes, VHB isn't anti-static, but it does do a *fair* job transferring heat (superb contrasted against Velcro), but it holds up well with those temps, and can be removed (albeit with a lot of patience and care) - without leaving behind a gooey mess.
That's what I'd do... Unless a 3rd party mounting bracket could be found on the cheap.
Oh wait - I forgot, I'm freak - I wouldn't use tape. I'd probably use some Dymax 429 Gel UV-activated epoxy, and glue it to the computer using that. But how many people have UV activated adhesives and high-power UV sources kicking around their homes? Not many I hope.