Many accordion players wear a glove on their left hand. This can make it easier to slide the hand up and down under the bass strap. It also helps keeping your accordion clean. What follows here is a description on how to make a dedicated accordion glove, using a sewing machine. The type of glove you see on the picture below has been put to the test for several years, and it has proven to work perfectly.
Due to the snug fit around the fingers, there isn't any tendency for the glove to slip off your hand. As you may see on the picture above, there is a padding tucked inside a small pocket on the palm side. The pocket is left open at the end that faces the place where the index finger sticks out from the glove. The padding is simply a piece that is cut out from a camping mattress (a.k.a. sleeping roll or bed roll). To make the padding thicker, two 7 mm thick pieces have been "glued" together with carpet tape (adhesive on both sides). It should, however, be possible to find a camping mattress that has just the right thickness; which would be 14 mm (0.55 inches).
The fabric being used is made of a synthetic material, and has just the right qualities: it's very thin, and thus it breathes, keeping your hand cool. It's also very low friction, making it easy to slide your hand up and down under the bass strap. The only concern here might be if you like to play your accordion while standing up. In this case the low friction could make it harder to manage the whole bass section in an efficient way.
The padding sits firmly in the pocket and has no tendency to slip out of it. The best position of the padding has been carefully tried out through experimentation. Although the padding makes your hand thicker, it should not be too difficult to slide your hand into position under the bass strap. You can also remove the padding and use the glove without it. Trim the padding with a pair of scissors to make its sides rounded. While the padding works great with the Stradella, it is also excellent when playing chromatic free bass since it gives your hand support against the bass cover plate, thus making it easier to keep your hand in a more relaxed position.
Here is a picture showing the glove turned inside out, with the temporarily removed padding placed on the fabric to show its relative location:
The fabric that has been used for this glove is washable, and the padding can be removed prior to washing. A disposable plastic glove (not the elastic rubber type, though) can be used as a template when cutting out the two halves from the cloth. The pocket should be sewed on to one of the halves (make sure you pick the correct one) before sewing the two halves together. Apply some stitches, by jogging the sewing machine on the same spot, to join the two halves in the places that will end up between your fingers. It can be somewhat difficult to sew the various parts because the fabric is so thin, so the whole procedure requires a bit of patience.
The fabric, shown above in about 38 x magnification, has lower friction in one direction. So, it might be a good idea to orient the fabric so that it becomes easy to slide the hand in the direction up/down.