I was never really happy with the print bed on my Select Mini. It was sagging in the middle and bent in other places. Getting PETG to stick was always a gamble and high prints regularly popped off near the end. However I didn't want a regular glass sheet because they tend to break all the time. I couldn't get an exactly matching borosilicate plate and having one cut would have been too expensive. So I bought the cheapest small borosilicate bed I could find in China and decided to design my own clamps. As you can see, the height adjustment screws are accessible but the problem is positive overlap on the sides and negative overlap front and back.
Using threaded rods and printed clips, my bed is now rock-solid and sits exactly where I want it. It does not move even when ripping off large prints. In the scad file you can adjust the glass dimensions, the offset from the front of the aluminum plate and select different types of fixings. There is a tiny bit of overhang on the clips to press down the plate. It doesn't collide with the nozzle on an unmodified Mini. I am using an E3DV6 lite (see below), so in my case I had to glue a little spacer (about 2mm) to the inside of the Y carriage where it hits the endstop.
The provided stl file is set up for a 140x140x3mm sheet exactly like mine. You need 4x M4 nuts in the front (I use 2x M4 nylon locking nuts facing outward) and 2x M4 nuts + washers in the back, as well as 2x 180mm M4 threaded rod. The design allows levelling the bed within limits (about 1 mm clearance in my case). You could add printed hand screws in the front or back for easier removal (should be doable without reprinting the clips), but I haven't felt the need yet.
With a shot of hairspray every now and then I never have problems getting PLA or PETG to stick. It is now basically a "press-start-and-go-do-something-else" kind of printer.
Don't forget the spacer for the Z limit switch. I use this, set to 3mm: https://www.thingiverse.com/thing:1808029
This is the hotend adapter I use with my E3DV6 lite:
EDIT: Removed a bug in the .scad file pointed out by Pete. Please use the fixed file if you want to play around.
Can be printed on the Select Mini. I went slow at 30mm/sec, using PETG. The design will probably only work for 0.4 nozzles, because the walls are rather thin.