Well, summoning, in my experience has two different avenues of approach. The first is a ritual calling to specific entities or spirits, like summoning circles or calls. The second is a little more passive, in which you leave offerings or symbols to attempt to draw certain beings to a particular area; a good example of this is leaving milk and honey in small circles to draw faeries or pixies. One could probably create a sigil encompassing a call to a being, something like "Faeries are drawn to this place." I doubt it would work for something like a deity's influence, but who knows?
Making your own from scratch is more for "summoning" deeper layers of your consciousness that in some believes are associated to "deities". Those can also be usefull for points of foccus.
But when it comes to actual summoning known or feared entities.. they wont give you the protection you need. Keys of Solomon have those seals because those are proven to be associated to those listed.
In any case... i strongly recommend not trying to cast any or all summoning spells untill your very adapt at such or under strict guidance from someone experienced. Summoning comes with great risks!