Learn to rest your thumb on the B string instead of a thumb rest. Very easy to learn, as you will play the B less than the others, most likely, unless you are in a gospel band. There are two advantages:
You can play anywhere from bridge to neck with all the different sounds in between.
Once you learn, all fivers and sixers pretty much play the same...you don't have to choose a bass because you like the pickup configuration as a thumb rest.
I second the rubber beer stopper as a straplock...I use washers myself. Cheap and non-invasive.