Harber Freight has an 8" swivel caster that can lock both rotation of the tire and also locks swivel action and it fits the standard bolt on pattern on most tail Dolly's. Bit on the heavy side at 9#, but the nylon Libelle tail wheel fits perfectly, so overall weight remains about the same. Many times it would be easier to leave the tail dolly on, just lock it to prevent any movement and on ships like the Genesis-2, leaving the tail dolly on is mandatory because the ship sits at a very high angle of attack when the tail dolly is removed and a 5 knot wind has rolled the ship into the sailplane behind it on the grid. $24.99 at Harbor Freight.
JJ