Andy,
Sorry to hear about your collections problems.
I think part of the problem may be the 8 month wait for a custom order. Circumstances change and customers loose interest. However, even setting aside the maker's anti-deposit culture for a moment, I can understand not wanting to take a deposit with that kind of lead time. The accounting problem alone would be nightmarrish.
On the other hand, you can't reasonably be expected to go into your own pocket to build a custom order either. Especially since you can't count on being able to sell it later to make yourself whole if the customer doesn't come through with the money -- what if there is no market for what the customer asked for (regardless of how well you executed it)? Your basically stuck financing somone else's design at that point.
Would it be appropriate (and acceptable among makers?) to protect yourself by contacting the customer when the order comes up to be made, confirming the details and whether there are any last minute changes, and requiring a non-refundable payment of say 50% of the quoted cost of the knife (to offset the cost of materials, etc.) prior to your commencing work? At least that way you will know if the customer is "real" before you commit time and materials. If you can't reach him or her, and/or do not receive payment, you just move on to the next one. Also, to the extent you weed out bad orders, you'll probably be able to shorten the wait time for better (read: "paying") customers.
By the way, . . .ahem . . . in the interest of full disclosure . . . . I have a Guardless Ladyfinger on order, scheduled for delivery sometime around Christmas, so I'm both calling in fire on my own position here, and also being admittedly a bit self-serving

I'm enjoying the Blue Terrasaur I snapped up a couple of weeks ago, in the meantime, btw.
Hope the show goes well!