feet & casters & handcarts
feet
Unless you use closed cell foam under and between enclosures, make sure you use rubber feet. They provide non-vibrating modular stacking not truly provided by "locking" chevron corners, and provide grip and damping on various surfaces, and protection for finishes. They also provide enclosure leveling when tilt-back casters are used in back.
or SpeakerHardware.com - 1" high by 1" diameter foot
Another little item that comes in handy is the folding cab leg. On a 12/6 these could concievably be used as handles as well, with handle cutouts relocated to achieve desired angles and balance. Anyway, they give a pretty good angle of tilt!
weak and lame
I almost giggle like a schoolgirl every time I see a rack or cab with teensy brittle wheels and rattle-prone pull-up handles. Sissy wheels such as these are ridiculously ineffective on pavement breaks and curbs - let alone on grass, ice or even peanut shells on a bar-room floor. They break easily and often get small gravel stuck up in the nacelles which impedes rolling. On a small enclosure luggage wheels are superfluous; on a larger one they are accident-prone.
Proper caster wheels should be 2.75" or 3" or larger if you actually want things to roll well and handle weight and stress.
real casters
I'm not much of a fan of plug-in casters either, though a little duct tape around the shaft can keep them from falling out of the sockets so easily. I question whether releaseable-plate casters are worth the extra cost and weight for items under 60 pounds (like fEARful 12/6 or 15/6 cabs) - though this this type does come in handy if you want to remove just the back wheels, which is one way to get tilt-back cab monitoring.
But instead, generally I favor permanently-attached 3" wheel sets if casters are even considered necessary. Casters larger than that weigh a lot more so on an easily-hefted cab that's going to be hauled up and down stairs, I stick with 3" which rolls nicely yet doesn't weight so much. Here is one of several casters that's available at SpeakerHardware.com.
tilt-back casters
Below are what I recommend for the taller fEARful 1212/6 and 1515/66 enclosures, to be used in conjunction with tilt-back handles. Make sure you space the wheels widely at the bottom so that the cab does not want to tip to the side when being rolled.
handcarts
Also consider dollies or hand-trucks if you move a lot of gear. Dollies are nice because they can be placed under PA racks at the gig - but they stink at negotiating terrain. Hand trucks excel there. There are quite a few products that are light and quickly fold up for storage. Many can negotiate stairs or be used like wagons. Since fEARfuls themselves are light, I prefer very light extensible units with long shelfs and larger wheels (5"-8"), like the one below, which is as cheap as I would consider going:
You can also sometimes find this type at Costco for $22 or so, and places like Lowes have similar products. Generally as cost goes up so does feature-set, and maybe durability. But this cart is sturdy - a great value. Also you can connect bungie or quick-release nylon web straps to them to hold enclosures and racks securely in any circumstances. I've even gigged with these still behind the rig. Easy in, easy out.
