I HIGHLY recommend that anybody who has family or friends who are interested in these things (kids i mean) just take a few minutes and build one. They're super cheap, and super easy. Go to your friendly local makerspace if you don't have the tools to do it yourself.
Do you have instructions for this that you've used with success?
Step 2: Insert a bearing of the correct size, aiming for an interference-fit. If the hole was too big, apply glue.