Kubota BX-24 - other goodies listed in my profile.
There are several threads about this very topic, with project plans, photos, etc. Try several phrases such as 'backhoe dolly" in the 'search' boxes, and they will surely pop up.
Then, look in the "Similar Threads" section at the end of those you find, and even more will turn up.
I built a dolly for my backhoe off my 2620 using two furniture moving dollies and a piece of pine about 24inches by four feet. This allowed me to move the bh around the shop (slowly) to keep it out of my way. Because of the weight I used a 2x4 as a jack to move it as my old back isn't what it used to be. I have a 3350 now with just a loader plus plow and blower and will be making a dolly for the loader using the same technique. The blower and a plow as well as the front sub frame and ratchet rake live one furniture dollies now.
I bought the BXpanded loader dolly and it's fine but as noted above a little pricey. However, I built dollies for my rear blade, chipper, snow blower, and will build my own BH dolly...all from 2X4s and plywood. It's not that difficult a project and if you get decent castors, they will work just as well as the BXpanded unit. Keep in mind how infrequently these things get moved around. They don't need to be works of art just sturdy and serviceable. Another benefit of a wood dolly is that you are less likey to ding paint off your attachment.