I used trunk bags for a few years until a few weeks ago, when I finally gave in to my reduced flexibility. For some reason, I can mount my bike easily, but I have some trouble getting my leg over the trunk bag when dismounting. I've started using Banjo Bros's Medium Handlebar bag, and I really like and recommend it. (Bontrager and J and D make similar bags, but IMO the Banjo Bros bag is better and cost roughly 30% less.)
I've used 2 cheap trunk bags. The first was
https://www.ebay.com/itm/TopSun-Rosw...IAAOSwrHhc6GCA. It's OK if you really need to save money, but even though I'm on a fixed income, after half a season, I thought something more solid was worth the cost. The foam in particular is very soft. It's wider than my rack, and it sagged on the sides.
The 2nd was
https://www.jandd.com/detail.asp?PRODUCT_ID=FRR-B - this is a substantial, durable bag, IMO. For $11 more, though, I wish I had gotten this:
https://www.jandd.com/detail.asp?PRODUCT_ID=FRRPE - the cord on the top of the bag would have been useful.
If you're willing and able to spend more than $50, you've got many more options, of course.