Why do we scale HSPU with boxes or dumbbells? Range of motion!! If you stack abmats and work against the wall, that can be fun, but you're never going to get better at full range of motion HSPUs!
That's why we always recommend using boxes or dumbbells to train the full movement. Have you been stuck at the same scale for a while? Maybe time to train some negatives (kicking up on the wall and lowering yourself as slowly as possible to the floor each rep).
The combo that helped me improve my HSPU was doing half negatives and half off a box in a pike position. Both scales train the full range but in a different way. Today's workout would be a good opportunity to try this out! Alternate between negatives on the wall and HSPU off a box.
Lastly, if you have kipping HSPU but struggle with strict HSPU, you should try doing some strict HSPU either on the wall or off the box! You could alternate between kipping HSPU and strict HSPU against the wall or off a box.
Try something different than your norm and challenge yourself in a new way!