Our child seems to enjoy being pushed around in a box. (My wife says I should point out the diaper box was Mark I.)
So I used scraps I had around the house and made this. (I collect plastic scraps for other projects, so I have lots of quality plastic stacked in the garage.)
Step 1: Construction
Baby Slider Mark II. 7 hours from scraps to here. Plastic is from plastic machining company scrap bin (the black sides were used as found). The base is scrap particle board. All side bolts are 2.25". All corners are rounded. The wheels are from a large Pelican case that someone borrowed and never returned.
Made using table saw and drill and socket wrench and vice, nothing else. Okay, there was also a ruler, marker and a face shield involved.
If there's interest, comment and I'll work up full instructions.