Cross-posted at RP, but this is too fun to keep in one place.

Check out this 38mm forward motor retainer I got at Home Depot this evening, for $2.97:

I'm sure an enterprising person could also figure out how to attach a shock cord to this.

Here you can see better how it works:

And you get a free extra-big wing nut as a bonus. If you splurge and buy two for about $6, you don't even need to belt-sand off the flange from the larger piece like I did.

So to review:

Infinitely adjustable motor retention and positioning system.
Requires no tools to install
Requires no glue to install
Puts no holes in your airframe
Use whatever length 38mm diameter motor you want
Comes compatible with a 38mm tapped forward closure right out of the plastic bag
Very strong and secure
Lets you use a flush rear closure for reduced drag.
No more friction fitting for 38mm!
Very, very strong. Did I say strong? Very strong.
Costs about $2.97 - $5.94
Available just about everywhere.

Hat tip to our own Ed, who made me aware of the concept in post #15 of this thread:
www.rocketryplanet.com/forums/showthread.php?t=2409&page=2&highlight=raven, where it stayed safely tucked away untill I ran across this gizmo at Home Depot last weekend.