Stephen,
I do not have a Mamiya 645 Pro, but I have a Hasselblad that I carry in a top loading pack. I do not like the packs that have a back that can let all the equipment spill on the ground.
The pack I use, the 750, puts the weight on the hips and holds my Hasselblad, the PME finder [think prism], two film backs, 50mm lens, 80mm lens, 150mm lens, 250mm lens, filters, film, tripod and filters. Now I wish I had the 752.
Take a look at
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Model 750 Photographer's Daypack
Internal Dimensions: 11¼ W x 6 D x 7 H
29 x 15 x 18 cm
External Dimensions: 17⅜ W x 9½ D x 14⅞ H
44 x 24 x 38 cm
Weight: 2 lbs. 10 oz. (1188 g)
Colors:
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The Photographer's Daypack was designed for one camera with attached lens and motor drive in Tamrac's patented
Lens-Bridge® with
Lens-Gate® divider system. This bag looks and functions like a daypack, yet the lower half is a completely foam-padded and compartmentalized camera bag. Adjustable foam-padded compartments protect three to four lenses, a flash and accessories. Additional compartments and pockets provide organization and extra space for your travel requirements. The large, quick-release top flap provides complete coverage and has a weather-flapped, zippered pocket that is perfect for maps and tickets. The thick foam padded backpack harness is contoured to spread the weight across the shoulders and back. The quick-release sternum strap keeps the shoulder straps in place. A nylon Tuck-A-Way™ waist strap keeps the pack from bouncing and tucks into a bottom compartment when not in use. Adjustable straps hold a small tripod. Constructed of fully-coated PowerGrid™ Cordura®, the camera compartment is padded with thick, closed-cell foam and the bottom has a strong plastic platform beneath the foam.
Model 752 Super Photo Daypack
Internal Dimensions: 13 W x 6 D x 8 H
33 x 15 x 20 cm
External Dimensions: 17 W x 9½ D x 16½ H
43 x 24 x 42 cm
Weight: 3 lbs. 10 oz. (1644 g)
Colors:
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The Super Photo Daypack is a larger, professional sized version of Tamrac's model 750 Photographer's Daypack. Safe, comfortable to carry and easy to use, this is the perfect camera bag for the traveling photographer with two camera bodies, four or five lenses and lots of accessories and personal items. It looks and functions like a daypack, yet the lower half is a completely foam-padded and compartmentalized camera bag. Additional compartments and pockets provide organization and extra space for your travel requirements. The large, quick-release top flap provides complete coverage and has a weather-flapped, zippered pocket that is perfect for maps and tickets. The thick, foam-padded backpack harness is contoured to spread the weight across the shoulders and back. A nylon Tuck-A-Way™ waist strap keeps the pack from bouncing and tucks into a bottom compartment when not in use. Adjustable straps hold a small tripod. Constructed of fully-coated PowerGrid™ Cordura®, the camera compartment is padded with thick, closed-cell foam and the bottom has a strong plastic platform beneath the foam.
I hope this helps you.
Steve