This patch automatically centers if only one window is open in the deck layout (and that window can't make use of the whole screen)
If there are multiple windows and the master window does not take up all it's alloted space, everything will be adjusted so no empty space is created. (only if there is only one master)
--- dwm.c	2020-08-24 09:43:01.749752454 +0200
+++ dwm.c	2020-08-24 09:43:23.786418316 +0200
@@ -673,15 +673,28 @@

 void
 deck(Monitor *m) {
-	unsigned int i, n, h, mw, my;
+	unsigned int i, n, h, mw, my, altw, nx;
 	Client *c;

 	for(n = 0, c = nexttiled(m->clients); c; c = nexttiled(c->next), n++);
 	if(n == 0)
 		return;
-
+	if (n == 1){
+		c = nexttiled(m->clients);
+		altw = WIDTH(c) * m->wh / HEIGHT(c); /*use max window height but preserve aspect ratio */
+		if ( c->mina > 0 && altw < c->mon->mw ){
+			nx = m->mx + (m->mw - altw ) / 2;
+			resize(c, nx , c->y, altw , c->h, 0);
+			return;
+		}
+	}
 	if(n > m->nmaster) {
 		mw = m->nmaster ? m->ww * m->mfact : 0;
+		if (m->nmaster == 1){
+			c = nexttiled(m->clients);
+			altw = WIDTH(c) * m->wh / HEIGHT(c); /*use max window height but preserve aspect ratio */
+			mw = c->mina > 0 && altw < mw ? altw : mw;
+		}
 		snprintf(m->ltsymbol, sizeof m->ltsymbol, "[%d]", n - m->nmaster);
 	}
 	else