I always have Protector fill all my bags I use and sell. They are very good at it and the Addie's shipping cost is offset by my time filling. I ask the not to fill the bags too tight. They are a great company to work with!
Sounds like a great ides. Post a photo when you get it filled.

High Noon

This works pretty good for filling eared bags. (You need to Login or Register to view media files and links)

QuestionableMaynard
I think you will find you get more consistent accuracy, shot-to-shot with bags that are filled rock hard rather than soft. But that's my experience, I'm sure this is a point on which there is no universal agreement.
If you are considering competition, the rules will answer your question about filling sand bags. Following are pertinent extracts from the ASSRA and ISSA rules.
ASSRA sand bags:
Section 2.91 Sandbags and Rests
A sandbag or shaped rear rest sand filled bag made of leather or canvas may be used under the toe of the buttstock. No other device or support is permitted for the toe of the rifle. V-blocks or rests, guides, ways, or any such devices which may aid in alignment and realignment of the rifle, are prohibited. An anti-cant device (bob-sled) similar to the Pope style muzzle rest or the one described in Mann’s book is permitted. Also a spirit level or sight attached to the barrel is permitted. A simple stop may be attached to the front rest as an aid in realigning the rifle for the next shot. This may be contacted by the muzzle rest or the forearm at the discretion of the shooter if used at all. The front rest assembly and rear bag may not be connected mechanically or tied together. The shooting bench top surface is the only common connection between the two that will be allowed.

ISSA sand bags:
8.5 Sandbags must contain a nonmetallic substance and the use of Velcro or other attachments between the bag and the bench is prohibited.
8.6 All devices or equipment which may facilitate shooting and are contrary to the spirit of these rules and regulations are forbidden. The responsibility shall be upon the competitor to submit questionable equipment for inspection for an approval by the Schuetzenmeister prior to the match.

Try Sinclair or Brownells.

Sinclair International : (You need to Login or Register to view media files and links) carries the heavy black sand.

I'm particularly interested in how your new bag supports the rifle while breach seating.  I've been using a low eared bunny bag to stop the cheekpiece interference, along with a v-notch front barrel rest.  It's a been a challenging balancing act to keep the rifle from falling off the bag while loading.  I suppose I could use a sled but I'm happy with the results using the barrel rest I have.

Thanks,
JS

Stay away from that pirate Lester Bruno.

40 Rod