Alternatives to the debris shelter?

All the survival training I've ever been a part of indicates that if you don't have a tarp or tent or other man-made shelter materials, you need to build a debris shelter. I have experimented with debris shelters on several occasions, in all seasons. When it's hot, I find the purpose of the shelter is to keep out mosquitoes. Otherwise there is no way to get any sleep. When it's cold, even a little cold - 50 degrees F - the purpose is to keep you warm. If it's precipitating, a well-built shelter will keep out the moisture.

The problem is, it takes FOREVER to build a good debris shelter (for a quick description of the process, go to Survival for kids. Scroll down to "Day 2: Shelter".). Worse, it takes A TON of energy. Spending tons of energy, sweating and scraping up your arms for several hours doesn't seem like a good survival strategy to me. Especially if you don't have food or water. On the other hand, if building the debris shelter is the only way to insure that you get a solid night's sleep and that you don't get dangerously cold, then...

But it's an open question in my mind, one that I intend to test over the next few months. Recently I was canoe-camping with some friends in the wilderness of northern Minnesota. One buddy and I decided to build  a debris shelter even though we had tents. After about 2 hours, we had a rough, inadequate shelter built. It had yet to be insulated on the inside, and the outer insulation was not sufficient to keep out the mosquitoes. I estimated it would take another 2 hours to collect enough insulation. And the shelter could only house one of us! We sort of gave up and just cooked dinner, retiring to our tents that night. In past experiences, I've built adequate shelters, but was totally exhausted from the effort.

I took two lessons from this and other shelter building experiences. First, if you want to really learn shelter building, don't bring a tent with you. You'll be more motivated. Second, I need to explore other options for quick, primitive shelter, where you do not have a saw or large knife. 

I have a few ideas I want to test out. 

The most obvious quick shelter would be to find a cave or similar natural structure with which I could quickly stuff with leaves and bed down for the night. I know of at least one cave around here where I intend to try that method.

But what if you can't find a cave? I have two ideas to test out. 

One is to cut or break a digging stick and dig out a big cavity in the ground. Stuff it with leaves and cover it with sticks and more leaves. This would require loose, sandy soil which is not too difficult to dig up. The big questions I need to answer about this proposed method are: (1) does it save any effort as compared to the traditional debris shelter? and (2) is the shelter effective for blocking mosquitoes and/or cold?

The other is to either (a) build a horse shoe shape from large rocks or (b) find two perfectly placed adjacent downed trees. Either way, stuff the cavity with leaves and cover with sticks and more leaves. Obviously the feasibility of this method would depend on availability of rocks or downed perfectly placed trees. The same questions need to be answered as for the ground shelter above.

Stay tuned for results. In the meantime, please post any relevant experiences to comments. Or, inquire about guest posting...