I'm surprised no one has mentioned this yet: There are SMBs that have an opening at the bottom that allows you to fill it with air from your 2nd stage regulator (e.g. I fill mine by doing a normal exhale and letting the bubbles from my reg exhaust go into the bottom of my SMB to fill it.
Now this all depends on the depth your're launching your DSMB from. If you're deep then this can be okay, however in my experience the only people I've witness using this method are DM's, and they generally launch from 10m or less, meaning that you get an DSMB at the surface that has little air in.
Although I carry 3 DSMB's my weapon of choice is my AP 6 footer that's primary filled via a crack bottle which is attached (Bottle refilled from your full tank at the surface). Twist open the valve and off she flys and arrives at the surface fully inflated and stays that way until you dump the air after use.
Two examples where is proved its worth:
On a dive in Thailand at the start of the Safety stop we got the Hull sign (lots of dive boats on that site had arrived after we dropped in) We had to head down - despite 2 DM's having sent up their DSMB (inflated via their 2nd stage exhaust). This happened a second time .. and a third - at which time I unleashed my AP DSMB which must have broken the surface like a sub launched missile - The boats saw that!
Second - in the Philippines - our group caught a drift current (my buddy and I thought it was fab, the other pair not so much) DM shot his DSMB as did I. Had 4 foot swells on the surface, boat a distance away - I could hold mine by the bottom above my head and wave it as it's a still tube - the boat saw it (mean while the DM has a small somewhat soft DSMB at the surface)
Of course if you're not experienced in launching DSMB's them if you got snagged as it went, that might be bad.
It can also be inflated via 2nd stage - I presume but stand to be corrected that 2nd stage filling via the purge in cold waters might get you a free flow?) I had to do this last weekend. On launching mine it was apparent I hadn't screwed the bottle all the way in, so was surrounded by lots of bubbles, a DSMB going nowhere and a buddy laughing her head off. Agine the internal valves work well and on the surface it was fully inflated and rigid
For the record my other two DSMBs are a conventional Oral/LPI/2nd stage one - but Yellow, and a monster 10 footer you inflate at the surface, as I predominately dive in remote blue water locations, where if I'm swept off the site I want the boat to see me. This along with the AP can be used as a surface float for 2 divers.